It is the long term objective of this study to investigate the mechanics of fetal maternal water exchange and the partition of fetal water among the intra- and extrafetal water compartments. 1. The measurement of the hydraulic conductivity of the placental barrier. 2. The measurement of the osmotic pressure that is developed by the common solutes of fetal and maternal plasmas, i.e. measurement of their "average" placental reflection coefficients. 3. To investigate whether any osmotically important solute is acquired by the fetus in effective quantity, either by active transplacental exchange, or by construction from smaller building blocks and, if so, to investigate how its acquisition is modulated in response to an experimental disturbance of the osmotic equilibrium. 4. The determination of the extent of hydraulic water transfer between fetal plasma and amniotic fluid. 5. The measurement of the diffusional permability between the fetal plasma and the amniotic and allantoic fluids of the commonly occurring fetal plasma solutes. 6. The determination of the origin of the transplacental electrical potential and its effect on the steady state distribution of ionic solutes between fetus, amniotic (and allantoic) fluids and maternal plasmas.